The present invention relates to an improved silver-cadmium oxide material of the type suited for use as electrical contact material, and to a method of making the same.
Silver-cadmium oxide strips for electrical contact material are well known in the art and are conventionally made by heating a silver-cadmium alloy to an elevated temperature in an oxidizing atmosphere to "internally oxidize" the cadmium. Typically, an alloy of 5 to 30 weight percent cadmium with silver is utilized. Minor amounts of grain refining ingredients may also be in the alloy, as is known in the art. By heating the alloy, say at a temperature of 750.degree. C. for a period of six hours, the cadmium is oxidized to cadmium oxide and a material which contains cadmium oxide particles dispersed in the body of silver is obtained.
One difficulty is that during the oxidation of the cadmium to cadmium oxide, the cadmium oxide tends to migrate from the interior of the silver-cadmium strip or bar being oxidized towards the surfaces thereof, resulting in a cadmium oxide depleted zone in the interior of the material. Thus, the oxidized strip or bar has an interior "depleted" zone, i.e., an interior core which is silver-rich but poor in cadmium oxide. The area of the material adjacent the outer surfaces thereof is correspondingly rich in cadmium oxide. The occurrence of such depleted zones is disadvantageous inasmuch as when the material is placed in use, such as use as an electrical contact, mechanical and electrical arcing wear on the surfaces wears away the cadmium oxide-rich area and exposes an area depleted or at least significantly poorer in cadmium oxide.
The prior art has attempted to overcome the problem of the cadmium oxide depleted zone forming during oxidation of the silver-cadmium material by utilizing a powder metallurgy technique in which silver and cadmium powders are blended, pressed and sintered to form the silver-cadmium oxide material which is extruded into finished form. In another technique, cadmium oxide powder is blended with the silver powder and similiarly treated. In this case, pre-oxidizing of the cadmium avoids the migration of cadmium oxide which takes place during oxidation of the conventional silver-cadmium alloy. However, materials made by these powder metallurgy techniques have been found to have inferior integrity in use as electrical contact materials.
Another type of technique employed by the prior art in order to overcome the problem of formation of cadmium oxide depleted zones in the material, is to oxidize very thin strips or sheets of silver-cadmium material to form the cadmium oxide. Because of the thinness of the sheets, the relative size of the resultant depleted zone is smaller. A number of such oxidized thin sheets, usually twenty to thirty, are then bonded together by rolling to form a strip of material which has alternate layers or strata of cadmium oxide particle-rich areas (formerly surface and near-surface areas) interspersed with very thin depleted zone areas. However, silver-cadmium oxide contact material made in this manner has a non-uniform cadmium oxide particle distribution because of the above mentioned alternating layers of cadmium oxide rich and cadmium oxide depleted areas, as well as showing non-uniform cadmium oxide particle size distribution.
The problem of internal cadmium oxide depleted zones is recognized in the art, for example, see U.S. Pat. No. 3,545,067 (Haarbye et al). This patent discloses forming the bimetal electrical contact material by forming a silver-cadmium shot, internally oxidizing the shot and compacting the extruding the shot. The patent also discloses the known technique of hot roll bonding a fine silver backing sheet to the extruded strip.
U.S. Pat. No. 3,807,994 (E. M. Jost) discloses a method for making silver-cadmium oxide contact material in which the cadmium oxide depleted zone can be effectively eliminated, and discloses that this is attained by pressure bonding a layer of silver-cadmium alloy of selected, relatively high cadmium content between layers of silver-cadmium alloy of relatively lower cadmium content, so as to form a composite silver-cadmium alloy material. This composite material is, after assembly, heated in an oxygen containing atmosphere to internally oxidize the cadmium content of the composite material.
It is an object of the present invention to provide a novel process for producing a silver-cadmium oxide material and a novel product of such process, one in which the cadmium oxide depletion has been substantially eliminated and the product has excellent metallurgical integrity characteristic of a wrought product.